Recently, with the tendency for lighter and more compact electronic devices (electronics), the density of printed wiring boards has been increased. In response to the increase of the density, it is required that a curable resin composition for forming a resin insulating layer such as a solder resist be improved in performance and workability. With the tendency for lighter, more compact and higher performance of electronic equipment, semiconductor packages reduced in size and increased in number of pins have been put into practical use, and mass production thereof has been accelerated. To deal with the increase of the density, IC packages called e.g., QFP (quad flatpack package) and SOP (small outline package) have been replaced by newly introduced IC packages called e.g., BGA (ball grid array) and CSP (tip scale package). In a substrate for such a package and a printed wiring board for use in automobiles, solder resists are used. For the solder resists, various photosensitive resin compositions have been proposed in the art (see, for example, Patent Documents 1, 2).
In a printed wiring board having fine-pitch wiring patterns such as a package substrate, highly dense wiring patterns are formed mutually close to each other. For the reason, the frequency of short circuit and crosstalk noise occurring between lines of the wiring pattern may increase. Accordingly, high reliability for insulation is required for a high performance solder resist for use in package substrates.
In the meantime, electrical control of automobiles, particularly the driving portions has been accelerated. Under the circumstance, a printed wiring board is increasingly installed in locations having high environment temperature, such as an engine room and peripheral portions thereof. Depending on the installation locations, the printed wiring board for automobiles will be placed in high-temperature conditions of 80° C. to 150° C. for a long term period.
Accordingly, a high performance solder resist for use in an automobile printed wiring board is required to have high insulation reliability in high-temperature conditions.
Prior to forming a resin insulating layer such as a solder resist on a substrate, usually a pretreatment for roughening a surface is applied to the substrate in order to improve adhesiveness of the resin insulating layer to the substrate. However, the pretreatment method for a substrate varies depending on the use. In some cases where a strong roughening treatment is not applied, a sufficient anchor effect is not expected. Thus, it is necessary for the resin insulating layer to have sufficient adhesiveness to a substrate regardless of what pretreatment method is applied.
A wide variety of resin compositions having excellent adhesiveness have been known. For example, Patent Document 3 discloses a curable epoxy composition containing an imidazole-isocyanate adduct as an epoxy curing agent, as an adhesive. However, the adhesive composed of an epoxy resin and using such an imidazole-isocyanate adduct as a curing agent fails to satisfy various properties (e.g., resistance to plating, durability, insulating property) required for a resin insulating layer such as a solder resist. The Document 3 is not concerned with the formation of a solder resist pattern by light exposure.